Three-phase electric motors are frequently burned out or damaged by failures in the power lines of the power system resulting in reduced line voltages detected or sensed by accompanying motor protectors but are damaging to three-phase motors. Prior attempts to correct for all of the possible line failures in a three-phase power system in general have been generally complex and costly and in some instances have not been entirely satisfactory for all applications or all known line failure conditions.
As background, utility companies must supply a large number of single-phase loads from the same three-phase lines in rural areas and small industrial installations. As a result the utility companies are usually unable to install devices on these lines that cut off a complete power line. A failure in the power supply to the motor is what is commonly referred to as a "single-phase condition," which means that one of the phases or legs in the primary or secondary side of the step-down line transformer has failed. This failure can result in a blown primary fuse, primary line failure, a single breaker malfunction at the substation, or a blown secondary fuse or secondary line failure. Such a malfunction in the power line supplying power to the motor does not always result in a completely dead phase. In essence, an adequate voltge is produced through the line transformer windings with two fully energized supply lines to allow the motor to run under a low voltage condition. This creates heating problems of one winding of the motor causing the varnish to deteriorate rapidly and ultimately resulting in a burn-out. Another problem encountered for three-phase motors in rural areas is simply a dangerously low voltage condition. Three-phase motor installations also have different starting currents, for example two motors of the same horsepower but different running speeds where one may require a higher percentage of starting current than the other.
Accordingly, it is an object of the present invention to provide a novel and improved protector for three-phase motors that protects against single phasing conditions caused by secondary or primary line failures.
Another object of this invention is to provide a simple protective device that can be readily applied to all types of commonly used three-phase motors with no change in the motor control circuit and can be easily and readily installed as an auxiliary part of existing motor control circuits.
Yet another object of this invention is to provide a protector device for motors that can be field-set to each and every motor installation in the field and may be easily and quickly installed without requiring special experience.
A further object of this invention is to provide a device that is compact in size, can be installed adjacent to the magnetic starter or motor control circuit, does not take up any significant working space, and does not require a modification of the existing motor control circuit.
Yet a further object of this invention is to provide a motor control circuit that is suitable for the commonly used line transformer winding connections such as wye-delta or delta-wye and is particularly effective for the commonly used wye-delta line transformer configuration having a high leg and a low leg.